The ability to adequately cool internal gas turbine engine components in next-generation commercial and military aircraft is of extreme importance to the aerospace industry as the demand for higher efficiency designs is constantly pushing engine operating temperatures higher. Advancements in cooling technology have allowed engines to run at increasingly high temperatures without a significant degradation of the structural properties of the component materials and recent developments continue to push the limits of material performance. Many of these developments involve precise control of the number, size, shape and location of small cooling holes on the surface of specific high temperature components, including blades, vanes, disks and compressor liners.
Laser-based machining processes such as cutting, welding and drilling are used in a variety of industries. In particular, the aerospace and land-based power generation industries rely heavily on laser processing of materials, e.g., for production of turbine engine components. Specifically, the use of lasers to drill small cooling holes in metallic and non-metallic components has dramatically increased in recent history and will continue as newer designs require more advanced cooling techniques in terms of the number, size, shape and precision of such cooling holes.
Pulsed laser drilling is used in creating cooling holes in high temperature engine components due to the potential for improved processing speed and positional accuracy as compared to conventional drilling techniques. In addition, recent advances in laser systems technology have enabled the creation of cooling holes of complex geometries that have improved airflow characteristics over conventional cylindrical holes. The trend towards higher efficiency engines coupled with new developments in laser system technologies is responsible, at least in part, for the increase in the number of cooling holes per engine, e.g., from 100,000 to more than 750,000 in proposed designs. As the production demand for laser-drilled cooling holes continues to grow, efficient and reliable process control techniques are needed in order to achieve the necessary manufacturing efficiency while ensuring quality conformance.
Efforts have been made to advance laser drilling technology, including specifically the potential collection and use of information of relevance and/or value in connection with laser drilling processes and related control systems. Thus, for example, the patent literature includes the following disclosures of relevance to laser drilling processes and/or related control systems:                1. Laser ablation feedback spectroscopy—US Published Patent App'n. # 20050061779        2. Method for securing a drilling process—US Published Patent App'n. # 20060237406        3. Method and apparatus for laser drilling—U.S. Pat. No. 6,054,673        4. Laser drilling system utilizing photoacoustic feedback—U.S. Pat. No. 4,504,727        5. Optical breakthrough sensor for laser drill—U.S. Pat. No. 5,026,964        6. Laser beam stop sensor and method for automatically detecting the presence of laser beam stop material using a laser beam stop sensor—U.S. Pat. No. 6,723,953        7. Fiber optic laser-induced breakdown spectroscopy sensor for molten material analysis—U.S. Pat. No. 6,762,835        8. Method for monitoring laser weld quality—U.S. Pat. No. 6,060,685        9. Method and apparatus for optically/acoustically monitoring laser materials processing—U.S. Pat. No. 5,045,669        10. Laser drilling breakthrough detector—U.S. Pat. No. 6,140,604Beyond the patent literature, the technical literature also includes teachings of relevance to laser drilling processes and/or related control systems, including:        1. Amoruso et. al, “Characterization of laser-ablation plasmas”, J. Phys. B: At. Mol. Opt. Phys, 32 (1999), R131-R172.        2. Szymanski et. al., “The spectroscopy of the plasma plume induced during laser welding of stainless steel”, J. Phys. D: Appl. Phys. No. 30 (1997) 3153.        3. Lober and Mazumder, “Spectroscopic diagnosis of plasma during laser processing of aluminum”, J. Phys. D: Appl. Phys. No. 40 (2007), p. 5917.        4. Duffey and Mazumder, “Spatially and temporally resolved temperature measurements of plasma generated in percussion drilling with a diode-pumped Nd: YAG laser”, J. of Applied Physics, Vol. 84, No. 8, 4122.        
Despite efforts to date, there is an unmet need for precise methods for process monitoring and process control for use in laser processing of materials. For example, due to technological limitations, hole completion and/or breakthrough is/are frequently detected and determined by trial-and-error. Alternatively, breakthrough may be detected and/or determined by a photodetector that is placed behind the workpiece being drilled. When the laser-drilled hole achieves full penetration through a component, the photodetector detects the “breakthrough” of the laser beam and calls for the next hole to be drilled. In addition, breakthrough detectors that are based on acoustic signatures originating during laser-material interaction are known. Other available breakthrough detection methods, which utilize the detection of radiation being emitted from the laser-material interaction zone, may be applicable to workpieces of complex or hollow geometries, where conventional breakthrough detectors cannot be used.
However, there are several shortcomings to the currently available methods for monitoring hole completion. For example, breakthrough detection by means of a photodetector requires that the workpiece material be single walled. Furthermore, such methods are not applicable to detecting breakthrough on components that are hollow where it is desired that only one wall be drilled. Hollow components are typically filled with a wax or wax-like material that absorbs the laser beam and protects against back-wall strike, which necessitates that the wax material be removed after completion of hole drilling.
Thus, despite efforts to date, a need remains for systems and methods that facilitate efficient and effective laser drilling processes. These and other needs are met according to the systems and methods of the present disclosure.